In order to increase the capacity of future mobile radio systems, it is necessary to considerably reduce the size of the cells used in present-day systems. Recent proposals have included the introduction of so-called microcells, by which is meant cells which each have a base station, such as in normal sized cells, although in this case the base station would merely cover an area of about 100-500 meters from. In this case, the vertical height of an erected base station antenna (or antennas) is the same as, for instance, street lighting. Typical use area are town and city centres in which fare density is high and where the surroundings are rich in disturbance sources. Cells of the aforesaid kind can be said to form in common a layer of cells (a microcell layer) over a given larger area.
For indoor use, a smaller cells than microcells can be used, so-called picocells. Each of these cells has a base station which covers an area of about 20-30 meters.
A given layer of cells, e.g. microcells, is covered by a number of normal cells, such that one normal cell (macrocell) will cover at least two microcells. Similarly, one microcell will cover the area of at least two picocells. A covering cell is called an umbrella cell. Thus, a given geographical area, for instance a densely populated area (Greater Stockholm) will be covered by macrocells, microcells and picocells. In this case, one or more macrocells may form the umbrella cells of a number of microcells.
A clear description of the terms used above is found in "ERICSSON REVIEW", No. 1, 1991 pp. 2-9.